Process and apparatus for industrial fractionation of polymers

Abstract

In a process for industrial fractionation of polymers using an extraction column and a homogeneous extraction agent a homogeneous feed is supplied to a region of the extraction column which is nearer the upper end thereof when the feed has a greater density than the extraction agent and which is nearer the lower end thereof when the feed has a lower density than the extraction agent. An apparatus for carrying out this process consists advantageously, instead of a pulsed sieve plate column as hitherto, of an unpulsed filling body column which leads to a better separation because a back mixing is avoided in favourable manner.

We claim: 
     
       1. Process for industrial fractionation of polymers using an extraction column and a homogeneous extraction agent, characterized by the following process steps: (a) a homogeneous feed is supplied to a region of the extraction column which is nearer to the upper end thereof when the feed has a greater density than the extraction agent and which is nearer to the lower end thereof when the feed has a lower density than the extraction agent;   (b) the homogenous extraction agent is supplied to a region of the extraction column nearer to the lower end when the feed has a greater density than the extraction agent and is supplied to a region nearer to the upper end when the feed has a lower density than the extraction agent, the column section lying below the upper supply point being held at a first temperature T 1  ;   (c) the flows of the feed and extraction agent are selected in such a manner that at the temperature T 1  two phases coexist and the desired distribution of the starting polymer amongst the two fractions obtained is ensured, a further transport of the polymer-poorer phase originating from the extraction agent being effected to an adjoining column region maintained at a temperature T 2  which is lower than the temperature T 1  when the polymer solubility decreases with the decreasing temperature and which is higher than the temperature T 1  when the polymer solubility decreases with increasing temperature;   (d) at the temperature T 2  a polymer-richer phase is separated which moves into the adjoining column region maintained at the temperature T 1 , is no longer completely dissolved there and is incorporated into the extraction process in such a manner that in the stationary state at the column end at which this phase emerges a gel phase is removed, which gel phase contains the molecular constituents desired in the one fraction;   (e) at the other column end the sol phase is discharged, which sol phase contains the molecular constituents desired in the other fraction; and   (f) the steps (a) to (e) are repeated with direct use of the gel fraction obtained as feed until the starting sample has been broken down into the desired number of fractions.   
     
     
       2. Process according to claim 1, characterized in that the temperature T 1  in the case of binary systems is selected up to 30° C. remote from the extreme of the turbidity curve in the interior of the miscibility gap and in the case of ternary systems in the temperature range of the liquid/liquid separation, T 2  being fixed up to 40° C. from T 1 , according to the temperature influences on the solubility of the polymers, both for the binary and for the ternary systems.   
     
     
       3. Process according to claim 1 characterized in that in the process step (f) when using mixed solvents their content of the thermodynamically better component is increased to such an extent that whilst retaining the temperature T 1  the more readily soluble constituents of the gel fraction of the previous process step used as feed become soluble, the temperature T 2  being closer to the temperature T 1  than in the preceding step due to the increased uniformity of the feed.   
     
     
       4. Process according to claim 1 characterized in that when using a single-component solvent in the process step (f) in the case of decreasing polymer solubility with decreasing temperature the temperature T 1  is raised and the temperature T 2  is set at a distance beneath the temperature T 1  which increases with decreasing amount of the mixing heat of the system.   
     
     
       5. Process according to claim 1 characterized in that when using a single-component solvent in the case of decreasing of the polymer solubility with increasing temperature the temperature T 1  is lowered and the temperature T 2  is selected correspondingly above said value.   
     
     
       6. Process according to claim 1 characterized in that the composition of the feed is defined by a point in the Gibbs phase triangle which lies outside the miscibility gap of the respective system for a given operating temperature.   
     
     
       7. Process according to claim 1 characterized in that when using a single-component solvent the temperature T 1  is selected so that for the case of solubility decrease of the polymer with decreasing temperature it lies beneath the maximum turbidity temperature and for the case of solubility decrease with increasing temperature it lies above the minimum turbidity temperature.   
     
     
       8. Process according to claim 1 characterized in that when using a mixed solvent the temperature T 1  is selected so that a stable liquid/liquid separation takes place to prevent crystallization, a chain breakdown of the polymer does not occur and the macroscopic phase separation takes place within the residence time of the phases in the apparatus.   
     
     
       9. Process according to claim 1 characterized in that the composition of the mixed solvent, depending on the fractionation, is selected so that the connecting line between the points in the Gibbs phase triangle which represent the composition of the feed and of the extraction agent intersects the associated miscibility gap.   
     
     
       10. Process according to claim 1 characterized in that the starting polymers are separated according to their chain length or substantially independently thereof according to their linear, branched or cyclic structure or according to their different content of chemically different structural elements.   
     
     
       11. Process according to claim 1 characterized in that in separation of polyacrylic acid in a concentrated aqueous solution the solution thereof in a mixture of water and an Mg salt or another analogously acting salt is used as feed and a concentrated aqueous solution of the Mg salt or of the analogous salt is used as extraction agent.   
     
     
       12. Process according to claim 1, characterized in that in separation of polyvinyl chloride (PVC) molecules, as feed composition a 100% mixture of THF/water/PVC of 75-85%/7.5-12.5%/7.5-12.% is selected, as extraction agent a 100% mixture of THF/water in the ratio 80-90%/10-20% is selected, a temperature of T 1  of 22°-26° C. is selected and an operating concentration of 4-6% PVC is selected. 
     
     
       13. Process according to claim 1, characterized in that in separation of polyisobutylene (PIB) molecules, as feed composition a 100% mixture of toluene/methylethyl ketone/PIB of 68-76%/13-17%/11-15% is selected, as extraction agent a 100% mixture of toluene/methylethyl ketone of 75-85%/15-25% is selected, a temperature T 1  of 20°-24° C. is selected and an operating concentration of 3-5% PIB is selected. 
     
     
       14. Process according to claim 1. characterized in that in the polymer charging of the fractionating column in the case of fractionation in the subcritical range a polymer concentration of 1/3 of the critical concentration is selected and in the case of the supercritical fractionation a concentration of 4/3 to 7/3 of the critical concentration is selected. 
     
     
       15. Process according to any one of the preceding claims for the processing or recycling and reuse of plastics. 
     
     
       16. Use of the polymer fractionated according to claim 1 as dispersing agent and/or as grinding agent for filler and/or minerals and/or pigments. 
     
     
       17. Use according to claim 16 for the paper industry. 
     
     
       18. Apparatus for carrying out the method according to claim 1 characterized by comprising an extraction column (10) having an elongated housing which comprises a lower section having a first temperature control means (17) and an adjoining upper second section having a second temperature control means (18) and a first inlet (15) for feed/extraction agent nearer to the upper end of the extraction column, a second inlet (14) for extraction agent/feed nearer to the lower end of the extraction column, a fraction discharge (16) at the upper end of the extraction column and a fraction discharge (12) at the lower end of the extraction column.   
     
     
       19. Apparatus according to claim 18, characterized in that the extraction column contains filling bodies (11).   
     
     
       20. Apparatus according to claim 19, characterized in that the filling bodies (11) are free of flushable and liquid-filled cavities and have a surface to which neither of the two phases adheres.   
     
     
       21. Apparatus according to claim 18, characterized in that the extraction column is a continously or periodically agitated column.   
     
     
       22. Apparatus according to claim 21, characterized in that the agitated column is a Scheibel column.   
     
     
       23. Apparatus according to claim 18, characterized in that the extraction column contains one or more mixer-separator stages.   
     
     
       24. Apparatus according to claim 18 characterized in that the extraction column (10) comprises instead of the two temperature control means (17) and (18) a temperature control means which generates a temperature gradient along the fractionating column.   
     
     
       25. A process according to claim 1, wherein the homogeneous feed comprises polymer molecules of different molecular weight and of chemically identical structural elements. 
     
     
       26. A process according to claim 25, wherein the polymer molecules are of a linear structure. 
     
     
       27. A process according to claim 1, wherein the homogenous feed is supplied to an intermediate region of the extraction column.